One pilot has died and another is seriously injured after a rocket belonging to billionaire Sir Richard Branson's space tourism venture crashed during a test flight, US authorities said.

Virgin Galactic's SpaceShipTwo rocket came down yesterday morning in the Mojave Desert in California.

An eyewitness reported that the craft exploded in flight after ignition.

A Virgin Galactic spokesman said the rocket "suffered a serious anomaly" after a test flight conducted by partner business Scaled Composites. California Highway Patrol said one person was dead and another had suffered a major injury.

The rocket is usually flown by two pilots.

Sir Richard wrote on Twitter: "Thoughts with all @virgingalactic & Scaled, thanks for all your messages of support. I'm flying to Mojave immediately to be with the team."

SpaceShipTwo has been under development at Mojave Air and Spaceport. It was designed to be carried into the air by the WhiteKnightTwo jet and then released before igniting its rocket to travel into space, before returning to Earth as a glider.

A Virgin Galactic spokesman said: "Our first concern is the status of the pilots."

He added: "We will work closely with relevant authorities to determine the cause of this accident and provide updates as soon as possible."

Photographer Ken Brown said he saw SpaceShipTwo get released from WhiteKnightTwo but explode after its rocket motor was ignited.